well if they have European users they basically have to make it opt-in

Not really. Depending on what you mean by European users, GDPR may not apply here (if GDPR was what you were alluding to).

Gdpr 100% applies here to any user residing in the EU, and as one I find it appalling that this is opt-out. Further, I couldn't find an option to delete my account, which is another clear violation of GDPR. I wonder how long before they get hit with a juicy fine.

Yeah, my understanding is that if they did not make an attempt to block EU citizens from using the site then GDPR does apply. The problem is that IIRC, when I was singing up they were explicitly serving only few cities in the US. Might be misremembering though, it was a while back.

Do you think American anti-hacking/DMCA/etc laws do not apply to people living overseas? (like Gary Mckinnon for example).

Corporations don't get to choose, either laws apply or they don't apply internationally.

If I recall correctly, the corporations that explicitly do not aim to serve EU citizens (and make reasonable attempts to block them) do not need to follow GDPR. Then there's a matter of enforcement - I don't think EU can do anything to a company that does not have any presence in the EU. IANAL, but I am an EU citizen living in the US so it would be great if I'm mistaken here. :-)